The present invention relates to sonar systems, and more particularly, to a parametric acoustic array sonar system for extracting the angular position of far field echo sources.
Most sonar systems operate by generating an electrical carrier signal. This carrier signal is then applied to a transducer, thereby producing an acoustic signal travelling along a given direction. When this transmitted acoustic signal strikes an object in its path, another acoustic signal is created. Its direction of travel will be back towards the transmitter. This in turn strikes a receiving transducer causing an electrical signal to be generated which can then be analyzed to determine the bearing of the reflecting object.
When the beam, or physical width of the transmitted sonar signal, is narrow enough, the bearing of the reflecting object is the same as the bearing of the return signal. In practice, however, this has proven to be most difficult to achieve. As the sonar beam travels through the medium, it disperses within a finite arc, with the beam's power diminishing with the distance from its center. As such, it has become the practice in the prior art to physically steer the beam. When a maximized return signal is received, the beam center will be directed at the reflecting object, and its bearing determinable as the same as the bearing of the transmitted signal. While the transmitted signal can be physically steered by mechanical positioning of the emitting transducer, the more common and accurate practice has been to use an array of mechanically fixed transducers, accomplishing a steered beam by manipulating the electrical signal applied to each element.
Thus, the prior art has sought a way to generate a narrow sonar beam without compromising its ability to be steered electronically. For example, the parametric array described in Westervelt, P. J., "Parametric Acoustic Array", J. Acoust. Soc. Vol. 35, No. 4, pp., 535-537, April 1963 was a device which generated a directional beam by amplitude modulating a highly collimated sound wave. This result was accomplished by simultaneous transmission of two high frequency waves which interact with the medium in a non-linear fashion to produce two new waves, one of which has a frequency equal to the sum of the original two frequencies and the other equal to the difference. The acoustic properties of the transmission medium were used to filter the transmitted signal so that only the narrowly focused difference wave, or lowest frequency component, survived over distance.
The accuracy of the transmitted sonar wave is a function of the accuracy of the electronic signals applied to each transducer of the array. As such, other prior art apparatuses have been concerned with techniques for precise control of the electronic signals. U.S. Pat. No. 3,324,425 issued June 6, 1967 to Brightman et al. makes use of the fact that if the phase of the signal applied to each transducer is delayed by a time interval equal to the distance between adjacent transducers in each row divided by the velocity of propagation in the medium surrounding the transducers, an end-fire beam substantially parallel to the array will be propagated. The azimuthal angle of the beam may then be controlled by adjusting the phase delay. This provided an improved phase control system using a time division multiplexed scheme based upon the use of high speed digital electronic counters and digital-to-analog converters. In a similar fashion, U.S. Pat. No. 4,190,818 issued Feb. 26, 1980 to Follin et al. teaches a means for steering the sonar beam by delaying the phase of the drive signal as a function of transducer position. It is representative of a class of prior art apparatuses which have improved beam control by using techniques other than amplitude modulation to generate the high frequency drive signal, such as pulse duration modulation, which can be used to produce the same non-linear combining effect described by Westervelt but by using a single high frequency carrier rather than two.